The present invention relates generally to a method and system for determining position by radio and more particularly to a method and system for measuring the baseline vector between a pair of points, such as survey marks, on Earth by radio interferometry using radio signals broadcast from earth orbiting satellites.
Some systems for determining position by radio make use of the directionality of the pattern of radiation of a transmitting or a receiving antenna. Other systems, including the present invention, do not rely upon directionality of any antenna. The present invention belongs to the general class of systems in which the position of a receiving antenna is determined by measuring the difference between the phases or the group delays, or both, of signals arriving from two or more different transmitting antennas whose positions are already known. If two transmission sources are synchronized, or if the departure from synchronism of two transmitters is known independently, then a measurement at the receiving site of the difference between the group delays of the signals arriving from the two sources determines that the receiver is located, in three dimensions, on a particular hyperboloid of revolution whose foci are the positions of the transmitters. If similar measurements at the same receiving site of signals from several different, suitably positioned, transmitters are combined, then the receiving position can be determined uniquely from the point of intersection of the corresponding hyperboloids.
Techniques of determining relative positions of different sites, one with respect to another, from measurements of the phase or the group delay differences between radio signals received simultaneously at those sites are also known in the art and are collectively referred to as techniques of geodesy by radio interferometry. The antennas at the separate sites are considered to form an interferometer, and the relative position vector that extends from one antenna to the other is called the baseline vector of the interferometer. The baseline, or relative-position, vector between two antennas can be determined usually with less uncertainty than the position of either individual antenna can be, because many potential sources of error tend to affect the measurements at both antennas nearly equally, and therefore tend to cancel when differences are taken between the two antennas. The technique of geodesy by microwave radio interferometry is known to provide an unmatched combination of accuracy, speed, and range for the determination of relative-position or interferometer "baseline" vectors. Such a determination may be based upon measurements of either the group-delay difference, or the phase difference, or of both differences between the signals received at the two ends of the baseline vector. Phase measurements are inherently more accurate than group delay measurements, but the interpretation of phase measurements is more complicated due to their intrinsic, integer-cycle, ambiguity. A general discussion of interferometric measurement techniques and the associated problems of interpretation is given in an article entitled "Radio Astrometry," appearing in Annual Reviews of Astronomy and Astrophysics, Vol. 14 (1976), pp. 197-214, by Charles C. Counselman III. A large collection of relevant technical papers appears in Conference Publication 2115 of the National Aeronautics and Space Administration, entitled "Radio Interferometry Techniques for Geodesy." Geodesy by radio interferometry has been practiced with radio signals emitted by various sources including natural ones such as quasars and artificial ones such as satellites of the NAVSTAR Global Positioning System (GPS).
As is known, there are presently about six GPS satellites orbiting Earth. The orbits of the satellites can be determined with an accuracy of about 2 meters. These satellites emit radio signals with wavelengths near 19.0 centimeters and also 24.4 centimeters. Provided that the integer cycle ambiguities of interferometric phase observations of these signals can be correctly resolved, the baseline vector extending from one antenna to another can be determined interferometrically with uncertainly much smaller than the wavelengths of the GPS transmissions. Determinations of three baselines, each baseline having a length of the order of 100 meters, by means of interferometric phase measurements of GPS signals were shown to have been accurate within about 1 centimeter, according to a report published in Eos (Transactions of the American Geophysical Union), Vol. 62, page 260, Apr. 28, 1981, by Charles C. Counselman III, S. A. Gourevitch, R. W. King, T. A. Herring, I. I. Shapiro, R. L. Greenspan, A. E. E. Rogers, A. R. Whitney, and R. J. Cappallo. The method employed in these interferometric baseline determinations was based on the known technique of direct crosscorrelation at a central location of the signals received separately but simultaneously at the two ends of each baseline.
In U.S. Pat. No. 4,170,776, there is described a system for measuring changes in a baseline vector between a pair of locations on earth using signals transmitted from the GPS satellites, in which the radio signals received at each location are precisely time tagged and then transmitted over telephone lines to a central location where a near real time phase comparison is made by crosscorrelating the two sets of signals. The system illustrated in the patent includes "dish" reflector type receiving antennas. Because the radio flux density of a GPS signal is small relative to the background noise level and because the bandwidth of a GPS signal greatly exceeds the bandwidth of a telephone line, the signal to noise ratio of the power transmitted over the telephone line from each location is small. It is largely for the purpose of raising this signal to noise ratio to a useful level that "dish" type antennas with large collecting areas are used in this system. Another important reason for the use of such antennas is that they are directive, so that signals arriving at the antenna otherwise than directly from the desired source are rejected.
Systems for measuring baseline vectors using other kinds of signals from Earth orbiting satellites are also known.
In an article entitled "Miniature Interferometer Terminals for Earth Surveying" (MITES), appearing in Bulletin Geodesique, Volume 53 (1979), pp. 139-163, by Charles C. Counselman III and Irwin I. Shapiro, there is described a proposed system for measuring baseline vectors using multi-frequency radio signals which would be broadcast from earth orbiting satellites, in which system the phases of the signals received are determined separately at each end of the baseline. That is, the signal received at one location is not crosscorrelated with the signal received at the other in order to determine the phase difference between the two signals. To resolve the phase ambiguity, the MITES system relies upon the combination of measurements at a set of up to ten frequencies suitably spaced between 1 and 2 GHz. Unfortunately, as far as is known, there are no satellites presently orbiting the earth which emit such signals.
Systems for measuring relative position using signals transmitted from sources other than artificial satellites are also known. One example of such a system using a lunar based transmission is also disclosed in U.S. Pat. No. 4,170,776.
Systems for measuring either a single position or a relative position using signals from sources other than orbiting satellites are also known. For example in an article by W. O. Henry, entitled "Some Developments in Loran," appearing in the Journal of Geophysical Research, vol. 65, pp. 506-513, Feb. 1960, there is described a system for determining a position (such as that of a ship at sea) using signals from ground based (stationary) transmitters. The system, known as the Loran-C navigation system, employs several-thousand-kilometer-long chains of synchronized transmitters stationed on the surface of the earth, with all transmitters using the same carrier frequency, 100 kiloHertz, and with each transmitter being modulated in amplitude by a unique, periodic, pattern of pulses. This pattern, which includes sign reversals of the amplitude, enables the receiver to distinguish between signals from different transmitters. A suitable combination of observations of more than one pair of transmitters can yield a determination of the receiver's position on the surface of the earth.
Another example of a system of this type is the Omega system which is described in an article by Pierce, entitled "Omega," appearing in IEEE Transactions on Aerospace and Electronic Systems, vol. AES-1, no. 3, pp. 206-215, Dec. 1965. In the Omega system, the phase differences of the signals received are measured rather than principally the group delays as in the Loran-C system. Because the frequencies employed in both the Loran-C and the Omega systems are very low, accuracies in position measurements with these systems are quite poor in comparison with the satellite systems mentioned.
The prior art also includes other methods of determining position and relative position by means of the Global Positioning System. The standard method, described for example in an article in Navigation, Volume 25, no. 2, (1978), pp. 121-146, by J. J. Spilker, Jr., and further described in several other articles appearing in the same issue of that journal, is based on measurements of the differences between the group delays, or the "times," of reception of the coded modulation of the GPS signals. In principle this method is a hyperbolic positioning method and is essentially similar to that of LORAN. The approximately 10 MHz bandwidth of the GPS modulation limits the accuracy of group-delay measurement and hence of position determination by the standard method to several tens of centimeters. Accuracy of the order of one centimeter is potentially available through the use of carrier phase measurements, as described for example in an article by J. D. Bossler, C. M. Goad, and P. L. Bender, entitled "Using the Global Positioning System for Geodetic Positioning," appearing in Bulletin Geodesique, vol. 54, no. 4, p. 553 (1980). However, every published method of using the GPS carrier phase for position determination has the disadvantage of requiring knowledge and use of the code modulation, which may be encrypted, or of requiring crosscorrelation of signals received at different locations, or of requiring the use of large antennas to raise the received signal to noise ratio and to suppress interference from reflected signals, or else the method suffers from more than one of these disadvantages. The present invention has none of these disadvantages.
In particular, the present invention requires no knowledge of the codes which modulate the GPS carriers, does not require crosscorrelation of a signal received at one location with a signal received at any other location, and does not require the use of a large or highly directional receiving antenna.